1. Field of the Invention
The present invention relates to an optical recording method and an optical recording apparatus for holographically recording information in which uneven consumption of the material through the thickness of a recording layer during recording is reduced so as to achieve high density recording and high multiple recording, an optical recording medium for recording by the optical recording method so that information recorded in a large volume and at high density is reproduced with efficiency, and an optical reproducing method and an optical reproducing apparatus for reproducing information recorded by the optical recording method in an efficient and excellent manner.
2. Description of the Related Art
Optical recording methods for holographically recording information on optical recording media generally utilize interference of an information beam (an object beam) carrying image information with reference beam within the optical recording media, interference fringes generated being written in the optical recording media. Such optical recording methods include a colinear system in which the information beam and the reference beam are emitted with their axes being coaxially aligned. In the colinear system, the information beam and the reference beam generate interference fringes, whereby image information is recorded in the recording layer. The recorded image information is reproduced by irradiating the optical recording medium with the same beam as the reference beam in the same direction as during the recording. Upon irradiation, diffracted light is generated from the interference fringes and received for producing the information.
Among techniques for increasing the optical information recording capacity is a multiple recording system in which the recording density of interference fringes is increased. Such recording systems include the shift-multiple recording, angle-multiple recording, wavelength-multiple recording, and phase-multiple recording.
Among them, the shift-multiple recording in which information is additionally recorded or overwritten on top of initially recorded information while shifting the emitted light or the optical recording medium little by little in a horizontal plane direction in relation to the recording layer is highly compatible with the disc recording such as CD and DVD in which the disc is rotated for recording and excellent in random accessing, thereby being used in the colinear system using a single lens for recording (see “NIKKEY ELECTRONICS,” Jan. 17, 2005, pp 105-114).
In the shift-multiple recording using the colinear system, as shown in FIG. 8, an information beam and a reference beam 34 emitted from a pick-up irradiate an optical recording medium 23 via an objective lens 12. The information beam and the reference beam 34 are collected in a form of an inverted cone of a specific size within a recording layer 4. Then, information is holographically recorded as an interference fringe of the inverted cone form in the recording layer 4. Following the holographic recording, the optical recording medium 23 or the pick-up is horizontally moved in the arrowed direction (the circumferential direction of the optical recording medium) in FIG. 8. Then, the information beam and the reference beam 34 irradiate a different region from the recorded region in the recording layer 4 for the subsequent holographic recording, sequentially conducting multiple recording.
However, the above multiple recording along with the horizontal movement allows for forming only one interference fringe through the thickness of the recording layer, limiting an increase in recording capacity.
In the meanwhile, a recording method using a combination of angle-multiple recording and shift-multiple recording in the two-beam interference system is disclosed (see the Japanese Patent Application Laid-Open (JP-A) No. 2004-177958). In this recording method, the incident angles of the information beam and the reference beam to the recording layer of a still optical recording medium are gradually changed for achieving multiple recording of information through the thickness of the recording layer. Following the multiple recording through the thickness, the optical recording medium is rotated for conducting multiple recording of information through the thickness in a new region of the photosensitive layer. In this way, multiple holographic recording is conducted not only in the horizontal direction but also through the thickness of the recording layer, thereby increasing the recording capacity.
In both of the above methods, as shown in FIG. 8, the information beam and the reference beam 34 are collected in the form of an inverted cone within the recording layer 4; therefore, the beam diameter varies according to the depth through the recording later 4. In other words, the beam diameter is large near the entrance and smaller near the focal point. Therefore, the light density is low in the area where the beam diameter is large (termed the upper part of the recording layer in some occasion hereafter) and increased as the beam diameter is reduced. Particularly, the light density is high near the vertex of the cone (termed the lower part of the recording layer in some occasion hereafter), where optical reaction is accelerated and the recording layer material such as monomers is exhausted. In contrast, the recording layer material is less consumed in the area where the beam diameter is large. Therefore, the recording layer material is subject to uneven consumption through the thickness of the recording layer.
Furthermore, no more recording is available in the area where the material is exhausted. Then, the next recording has to be moved to an area that does not overlap with that area, which results in not much recording in the horizontal direction. The recording layer material is wasted and the recording density is reduced. A limitation is imposed in increasing the recording capacity.
Therefore, an optical recording method and an optical recording apparatus for holographically recording information in which uneven consumption of the material through the thickness of the recording layer during recording is reduced so as to achieve a high recording density and an increased recording capacity, an optical recording medium for recording by the optical recording method so that information recorded in a large volume and at high density is reproduced with efficiency, and an optical reproducing method and an optical reproducing apparatus for reproducing information recorded by the optical recording method in an efficient and excellent manner have not been realized yet and currently demanded to be provided.